The invention relates to switched capacitor band gap reference circuits, and more particularly to "curvature correction" therefor.
The closest prior art is believed to include U.S. Pat. Nos. 5,059,820, 4,375,595 (Ulmer et al.), and U.S. Pat. No. 5,563,504 (Gilbert et al.), which disclose switched capacitor band gap reference circuits having no curvature compensation.
Silicon band-gap voltage reference circuits are widely used to provide precision internal reference voltages in integrated circuits. There are many implementations of band-gap reference circuits, most of which have been of a "continuous time" nature. That is, most of the known band-gap reference circuits are analog circuits that do not include switching circuitry. However, many CMOS integrated circuit structures include nearly ideal capacitors; this has led to development of accurate band-gap reference voltage circuits using "discrete time" switched capacitor circuits. The above mentioned patents disclose several such switched capacitor band-gap voltage reference circuits, in which ratios of nearly ideal capacitors determine reference voltage scaling factors.
Prior art FIG. 1 shows switched capacitor voltage reference circuit 1, which includes a conventional switched capacitor voltage reference circuit 2 as generally disclosed in above mentioned U.S. Pat. No. 5,059,820, with a conventional sample/hold and buffer output circuit 4. Briefly, the currents NI and I of current sources 16 and 18, respectively, are time-multiplexed during non-overlapping clock signals .phi.1 and .phi.2 to produce the current I.sub.1 flowing through diode-connected transistor Q1 so as to produce in a capacitor C2A a charge proportional to a V.sub.BE voltage of transistor Q1 when it is conducting current NI; that charge, which has a negative temperature coefficient, then is transferred to feedback capacitor C2. Then, a change in charge proportional to a .DELTA.V.sub.BE voltage which transistor Q1 undergoes when the current through it changes from NI to I is stored on capacitor C1; that charge, which is proportional to absolute temperature, then is transferred to feedback capacitor C2. The net charge transferred to feedback capacitor C2 is converted by operational amplifier 25 to the voltage V.sub.OUT on conductor 27. A conventional sample and hold circuit and buffer circuit 4 simply converts the voltage V.sub.OUT to a "continuous time" reference voltage V.sub.REF.
The circuit of prior art FIG. 1 and all of the known band-gap reference circuits perform, in essence, the summation of a base-emitter voltage V.sub.BE of a transistor which has a negative temperature coefficient with a PTAT (proportional to absolute temperature) voltage which has a positive temperature coefficient in order to achieve cancellation of the positive and negative temperature coefficients. However, all band-gap reference circuits also exhibit an inherent undesirable "curvature" in their reference voltage versus temperature characteristic, due to the non-linearity of the temperature coefficient of the V.sub.BE voltage of any bipolar transistor. So-called "curvature compensation" circuitry has been used in conjunction with the known "continuous time" band-gap voltage reference circuits in order to achieve reduction in such curvature, as shown in U.S. Pat. No. 5,519,308 (Gilbert et al.). However, the known prior curvature compensation circuits are compatible only with the known "continuous time" or analog band-gap voltage reference circuits, but have not been used with switched capacitor band-gap voltage reference circuits.